So, back to the question of libldap's TLS support... The alternate code
in HEAD allows multiple TLS implementations to be used at once. The idea
here was that a single libldap binary would be able to coexist with
multiple apps even if they explicitly used different TLS libraries
themselves. In practice I'm not sure that's so important, since probably
few apps are aware enough to even make that choice. The other downside
of this approach is that the meaning of SSL and SSL_CTX handles in
libldap changed (they had to be wrapped so we could insert a tag
identifying which implementation went with which handle).

One possibility here is to preserve the old TLS options for OpenSSL
only, and make them fail on the other implementations, and introduce new
TLS options just for manipulating the generic handles.

Another choice here is to keep the modular layout but still only support
one implementation at a time, chosen at compile time. Then we don't need
the identifying tag in each session and context handle.